The present invention relates to digital imaging, and more specifically to color correction by calibrating color or adjusting color balance, which may also be known as white balance, gray balance or neutral balance.
Because light sources have varying color temperatures, images captured by standard or digital cameras often exhibit a color cast, which causes an image of a subject to appear differently from an image of the subject captured under different lighting. One may adjust the color balance attribute to remove the color casts caused by light sources. Color balance may also be referred to as white balance, gray balance, or neutral balance.
Accurate color rendition is vital in digital imaging, and correct color balance is a key factor in achieving accurate color rendition. Achieving correct color balance is typically accomplished by applying a color temperature profile to a digital image after the image is captured. In cases where the color temperature of the light source is stable over time, this technique often works well. For a group of images taken under a stable light source, the correct color temperature profile need only be determined for the first image in the group, and can then be applied to all the remaining images in the group.
Achieving correct color balance for a group of images becomes more complicated if the color temperature of the light source changes from image to image. In this case, each image must be analyzed individually to determine the correct color temperature profile to apply.
Achieving correct color balance for an image or images illuminated by fluorescent lighting is also complicated. Fluorescent lights change color significantly and continuously at the frequency of the alternating current (“AC”) which powers them. For example, fluorescent lights in the United States may change in color at a 60 times per second cycle. Images captured using exposure times shorter than one complete AC cycle may show significant color shifts with respect to each other, and so will require different color temperature profiles for each image to achieve proper white balance. Worse still, if the exposure time is sufficiently short, the images will show a color variation within each image. For example, the top portion of the image will have a different color cast from the middle portion of the image, which will have a different color cast from the bottom portion. In such cases, there is no single color temperature profile which can be applied to the entire image to achieve correct color balance.